Emmanuel Macron solidified his position as the front-runner in the French presidential election, picking up a key endorsement as rival Francois Fillon's legal woes continued.

Former Prime Minister Manuel Valls said he will vote for Macron in the first round of the presidential race on April 24 instead of Socialist Benoit Hamon, setting off a crisis within France's oldest party.

Valls said the 39-year-old independent is best-placed to stop the National Front's Marine Le Pen and her bid to take France out of the euro.

With just over three weeks before the first round of voting, polls show Macron and Le Pen are most likely to qualify for the May 8 runoff while voters look set to eliminate both the main parties' candidates for the first time since France began directly electing its presidents 50 years ago.

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"One must not take any risks with the Republic," Valls said in an interview with RMC radio. "Therefore I'll vote for Emmanuel Macron. The interests of France are above that of the rules of any party or primary."

He said he fears Le Pen's support is under-reported by polls.

Hamon shot back angrily at Valls, calling his behaviour "dishonourable".

Macron who served as economy minister under Valls before quitting to run on his own, was speaking on another radio station when his former boss gave his endorsement.

Macron thanked Valls for his support but said he remained politically independent and committed to renewing the personalities and practices of French politics. Valls also said he didn't plan to work with Macron.

Hamon called on leftists from "social-democrats" to communists to back his "transformative and credible project," including those who now support rival leftist candidate Jean-Luc Melenchon, who is running slightly ahead of Hamon. On Monday, Hamon said on France 2 television that he's been "stabbed in the back" by Socialists who have endorsed Macron.

Melenchon rejected Hamon's call. "I am not going to give up on this journey to get involved in some sort of who-knows-what arrangement," Melenchon told 5000 supporters at a rally today in the Channel port of Le Havre. He said he'd never submit to the Socialist Party because it's "renounced the fight against trans-national finance".

Socialist President Francois Hollande decided not to run for re-election because of his record-low popularity, but the party primary exposed a deep divide between a leftist wing represented by Hamon and a rival faction, headed by Valls, that is more concerned about business and public order.

According to Bloomberg's composite of opinion polls, Macron would win 25 per cent of the vote and Le Pen 26 per cent on April 23. Fillon lags at 19 per cent, Melenchon at 14 per cent and Hamon at 10 per cent. Another six candidates share the remaining votes. Every poll has shown Macron easily winning the run-off over Le Pen.

With Hamon struggling and Le Pen cozying up to Russia's President Vladimir Putin as she threatens to upend the European Union, other Socialist heavyweights such as former Paris Mayor Bertrand Delanoe and Defence Minister Jean-Yves Le Drian have supported Macron.

Hamon supporters accused Valls of reneging on a promise to support whoever won the primary, with Arnaud Montebourg, a former industry minister, saying Valls "has no honour".

Valls said Hamon and Montebourg can't give lessons in loyalty because they both quit his government in 2014 and belonged to a group of Socialist lawmakers who proudly called themselves "les frondeurs," or rebels, and opposed many government laws.

- Bloomberg